The development in the 1990s of a high-brightness blue-light-emitting LED made possible the introduction of commercial high-efficiency, white-light-emitting LEDs that are usable in general lighting applications. The realization of white light from these monochromatic blue LEDs is achieved by employing phosphors which convert at least a portion of the shorter-wavelength blue light into longer green, yellow and red wavelengths. One phosphor system of considerable interest for such phosphor-conversion LEDs (pc-LEDs) is based on the family of cerium-activated garnets represented by the general formula A3B5O12:Ce, wherein A is Y, Sc, La, Gd, Lu, or Tb and B is Sc, Al or Ga. These garnet-based phosphors have a cubic lattice structure and absorb wavelengths in the range from 420 nm to 490 nm which means that they are excitable by radiation from a blue light source such as a blue LED. Other garnet phosphors such as (Y, La, Gd)Na2Mg2V3O12:Eu, Bi and Ca3Sc2Si3O12:Mg,Ce are also of interest for pc-LEDs because of their red-light emissions which may be used to improve the color rendering index (CRI) of white-emitting pc-LEDs.
Of the garnet phosphors, cerium-activated yttrium aluminum garnet, Y3Al5O12:Ce, (YAG:Ce), has seen widespread use in commercial white-emitting pc-LEDs. The YAG:Ce phosphor has been shown to be a very efficient converter for blue wavelengths generating a broad intense yellow emission band centered at about 575 nm. This intense yellow emission and the remaining unconverted blue light emitted by the LED combine to form a white light emission.
One drawback to using the YAG:Ce phosphor by itself in a pc-LED is that the white emission from the pc-LED has a high color temperature and relatively low color rendering index (CRI). One way to produce a warmer white light and increase CRI is to add a red-emitting phosphor. However, phosphor mixtures tend to have reduced efficacy because the phosphors interfere with one another due to energy transfer via overlapping emission and absorption as well as non-radiative processes. Another way to adjust the emission of the pc-LED is to change the elemental composition of the phosphor to increase output in the desired wavelength range. Unfortunately, this can also affect the efficiency of the phosphor, resulting in a lower efficacy LED.
Phosphor-conversion LEDs can also be used to produce single-color LEDs by fully converting blue or UV light emitted by an LED into another color such as green or red. This is desirable in some cases because the pc-LED efficacy is greater than that of the comparable monochromatic direct semiconductor LED. However, the range of colors that can be produced by full conversion is similarly limited by the ability to manipulate the composition of available phosphors.
Thus it would be an advantage for both white and single-color pc-LEDs to be able to adjust the emission colors of available garnet-based phosphors without having to change their composition.